The current offset has the scale already applied to it. The ABI
documentation defines the offset parameter as "offset to be added
to <type>[Y]_raw prior to scaling by <type>[Y]_scale in order to
obtain value in the <type> units as specified in <type>[Y]_raw
documentation"
The right value is obtained at 0 degrees Celsius by the formula provided
in the datasheet:
T = Tsens_t0 + (Tadc_t - Tadc_t0) / Tadc_res
where:
T = 0 degrees Celsius
Tsens_t0 (reference temperature) = 25 degrees Celsius
Tadc_t0 (16-bit format for Tsens_t0) = 17508
Tadc_res = 60.1 LSB/degree Celsius
The resulting offset is 16605.5, which has been truncated to 16005 to
provide an integer value with a precision loss smaller than the 1-LSB
measurement precision.
Fix the offset to apply its value prior to scaling.
Signed-off-by: Javier Carrasco <[email protected]>
---
drivers/iio/magnetometer/tmag5273.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/iio/magnetometer/tmag5273.c b/drivers/iio/magnetometer/tmag5273.c
index c5e5c4ad681e..d22ca39007b6 100644
--- a/drivers/iio/magnetometer/tmag5273.c
+++ b/drivers/iio/magnetometer/tmag5273.c
@@ -356,7 +356,7 @@ static int tmag5273_read_raw(struct iio_dev *indio_dev,
case IIO_CHAN_INFO_OFFSET:
switch (chan->type) {
case IIO_TEMP:
- *val = -266314;
+ *val = -16605;
return IIO_VAL_INT;
default:
return -EINVAL;
---
base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
change-id: 20231023-topic-tmag5273x1_temp_offset-17774cbce961
Best regards,
--
Javier Carrasco <[email protected]>
On Mon, 23 Oct 2023 11:50:02 +0200
Javier Carrasco <[email protected]> wrote:
> The current offset has the scale already applied to it. The ABI
> documentation defines the offset parameter as "offset to be added
> to <type>[Y]_raw prior to scaling by <type>[Y]_scale in order to
> obtain value in the <type> units as specified in <type>[Y]_raw
> documentation"
>
> The right value is obtained at 0 degrees Celsius by the formula provided
> in the datasheet:
>
> T = Tsens_t0 + (Tadc_t - Tadc_t0) / Tadc_res
So base units for temperature are milli degrees celsius.
T = 1000 * (25 + (adc - 17508) / 60.1)
T = 1000/60.1 * (25 * 60.1 + adc - 17508)
T = 10000/601 * (-16005.5 + adc)
So I think the maths is a little off..
>
> where:
> T = 0 degrees Celsius
> Tsens_t0 (reference temperature) = 25 degrees Celsius
> Tadc_t0 (16-bit format for Tsens_t0) = 17508
> Tadc_res = 60.1 LSB/degree Celsius
>
> The resulting offset is 16605.5, which has been truncated to 16005 to
Interesting - the truncated value you have looks good to be but that's
not matching with the resulting offset or the value below...
> provide an integer value with a precision loss smaller than the 1-LSB
> measurement precision.
>
> Fix the offset to apply its value prior to scaling.
>
> Signed-off-by: Javier Carrasco <[email protected]>
> ---
> drivers/iio/magnetometer/tmag5273.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/iio/magnetometer/tmag5273.c b/drivers/iio/magnetometer/tmag5273.c
> index c5e5c4ad681e..d22ca39007b6 100644
> --- a/drivers/iio/magnetometer/tmag5273.c
> +++ b/drivers/iio/magnetometer/tmag5273.c
> @@ -356,7 +356,7 @@ static int tmag5273_read_raw(struct iio_dev *indio_dev,
> case IIO_CHAN_INFO_OFFSET:
> switch (chan->type) {
> case IIO_TEMP:
> - *val = -266314;
> + *val = -16605;
> return IIO_VAL_INT;
> default:
> return -EINVAL;
>
> ---
> base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
> change-id: 20231023-topic-tmag5273x1_temp_offset-17774cbce961
>
> Best regards,
On 27.10.23 16:15, Jonathan Cameron wrote:
> On Mon, 23 Oct 2023 11:50:02 +0200
> Javier Carrasco <[email protected]> wrote:
>
>> The current offset has the scale already applied to it. The ABI
>> documentation defines the offset parameter as "offset to be added
>> to <type>[Y]_raw prior to scaling by <type>[Y]_scale in order to
>> obtain value in the <type> units as specified in <type>[Y]_raw
>> documentation"
>>
>> The right value is obtained at 0 degrees Celsius by the formula provided
>> in the datasheet:
>>
>> T = Tsens_t0 + (Tadc_t - Tadc_t0) / Tadc_res
>
> So base units for temperature are milli degrees celsius.
> T = 1000 * (25 + (adc - 17508) / 60.1)
>
> T = 1000/60.1 * (25 * 60.1 + adc - 17508)
> T = 10000/601 * (-16005.5 + adc)
> So I think the maths is a little off..
>>
>> where:
>> T = 0 degrees Celsius
>> Tsens_t0 (reference temperature) = 25 degrees Celsius
>> Tadc_t0 (16-bit format for Tsens_t0) = 17508
>> Tadc_res = 60.1 LSB/degree Celsius
>>
>> The resulting offset is 16605.5, which has been truncated to 16005 to
> Interesting - the truncated value you have looks good to be but that's
> not matching with the resulting offset or the value below...
>
You are right, there is a typo in the resulting offset and value below.
The right value is the truncated one I mentioned, which is the one
obtained with the formula.
In the case of milli degrees Celsius for T = 0 (milli or not does not
matter then):
0 = 1000 * (25 + (adc - 17508) / 60.1) =>
=> adc = -25*60.1 + 17508 = 16005.5
I will fix the value to use the one I actually obtained (16005)
Thank you.
>> provide an integer value with a precision loss smaller than the 1-LSB
>> measurement precision.
>>
>> Fix the offset to apply its value prior to scaling.
>>
>> Signed-off-by: Javier Carrasco <[email protected]>
>> ---
>> drivers/iio/magnetometer/tmag5273.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/iio/magnetometer/tmag5273.c b/drivers/iio/magnetometer/tmag5273.c
>> index c5e5c4ad681e..d22ca39007b6 100644
>> --- a/drivers/iio/magnetometer/tmag5273.c
>> +++ b/drivers/iio/magnetometer/tmag5273.c
>> @@ -356,7 +356,7 @@ static int tmag5273_read_raw(struct iio_dev *indio_dev,
>> case IIO_CHAN_INFO_OFFSET:
>> switch (chan->type) {
>> case IIO_TEMP:
>> - *val = -266314;
>> + *val = -16605;
>> return IIO_VAL_INT;
>> default:
>> return -EINVAL;
>>
>> ---
>> base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
>> change-id: 20231023-topic-tmag5273x1_temp_offset-17774cbce961
>>
>> Best regards,
>